Simplify and refactor element classes for nested Matrices/Vectors #36

Framework in its current state defines, from scratch, home-grown Vector2 and Matrix2 classes (implementations based on those in InfiniTAM).

This is unnecessary and difficult to expand to alternative element types, i.e. Vector3, Matrix4, etc. Instead, these should be just classes extending Eigen's own Vector3, Matrix2, etc., adding the missing functionality.

For instance, Vector2f_uv class could add a Vector2f(single_value) constructor, u() and v() member functions (getters), etc.